The provision of 34 Mbits television services on the synchronous network requires stringent control of low frequency phase at the desynchroniser. As 34 M video codecs linearly encode the colour sub carrier of the PAL signal, any phase disturbances introduced by the synchronous transport (e.g. SDH) network propagate through the codec and into the reconstituted video. There are two bounding parameters that the video network providers consider to be important to deliver an acceptable quality of service to customers. First the phase transients in the bit stream must be such that the colour sub carrier, at 4.43 MHz will have no greater than a 0.1 Hz per second rate of change. Second, the delay budget in a telephony link must be no more than 100 microseconds. The final parameter of interest in bounding the desynchroniser system is that the network requires a minimum of 30 s to set up a connection. This is very much a worst case scenario, and more common switch times are in the order of up to 30 minutes and more. The most difficult problem is that of eliminating phase disturbances arising from the arrival of the video pointers which are used to bring the net flow of information back to the original input rate. A number of techniques have been proposed to address this problem, but none has been entirely successful.
A desynchroniser which uses the synchronous property of SONET to diminish the effect of pointer variations on an output clock phase is described by S. Say in Bellcore Standard T1X1.6/88-026, Jul. 25, 1988, pages 2 to 9.